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Low-Frequency Current Observations in the Korea/Tsushima Strait*

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Low-Frequency Current Observations in the Korea/Tsushima Strait* JUNE 2002 TEAGUE ET AL. 1621 Low-Frequency Current Observations in the Korea/Tsushima Strait* W. J. T EAGUE,G.A.JACOBS,H.T.PERKINS, AND J. W. BOOK Naval Research Laboratory, Stennis Space Center, Mississippi K.-I. CHANG AND M.-S. SUK Korea Ocean Research and Development Institute, Seoul, Korea (Manuscript received 13 March 2001, in ®nal form 21 September 2001) ABSTRACT High resolution, continuous current measurements made in the Korea/Tsushima Strait between May 1999 and March 2000 are used to examine current variations having time periods longer than 2 days. Twelve bottom- mounted acoustic Doppler current pro®lers provide velocity pro®les along two sections: one section at the strait entrance southwest of Tsushima Island and the second section at the strait exit northeast of Tsushima Island. Additional measurements are provided by single moorings located between Korea and Tsushima Island and just north of Cheju Island in Cheju Strait. The two sections contain markedly different mean ¯ow regimes. A high velocity current core exists at the southwestern section along the western slope of the strait for the entire recording period. The ¯ow directly downstream of Tsushima Island contains large variability, and the ¯ow is disrupted to such an extent by the island that a countercurrent commonly exists in the lee of the island. The northeastern section is marked by strong spatial variability and a large seasonal signal but in the mean consists of two localized intense ¯ows concentrated near the Korea and Japan coasts. Peak nontidal currents exceed 70 cm s21 while total currents exceed 120 cm s 21. The estimated mean transport calculated from the southwest line is 2.7 Sv (Sv [ 106 m3 s21). EOF analyses indicate total transport variations in summer are due mainly to transport variations near the Korea coast. In winter, contributions to total transport variations are more uniformly distributed across the strait. 1. Introduction The western channel contains a closed topographic de- pression in its center, approximately 60 km long, 10 km Of the four straits that connect the Japan/East Sea wide, and maximum depth 200 m. Its position is ap- with adjacent seas (Korea, Tsugaru, Soya, and Tatar proximated in Fig. 1 by the 150 m closed contour west Straits), the Korea/Tsushima Strait provides the vast ma- and northwest of Tsushima Island. jority of in¯ow and thus is instrumental in controlling Long-term measurement of currents in the Korea/ the Japan/East Sea properties. The major in¯ux of sea- Tsushima Strait are very dif®cult to acquire due to the water entering the Japan/East Sea through this strait is intense level of ®shing and trawling (Kawatate et al. important in the transfer of dynamic, thermodynamic, 1988). Hence, such measurements are relatively scarce. biological, and passive tracer properties into the Japan/ Many short-term current measurements have shown East Sea. This strait is effectively a shallow, connecting strong currents in addition to strong tidal components channel between the broad, shallow East China Sea (Mizuno et al. 1989; Egawa et al. 1993; Isobe et al. (depths of approximately 50 to 1000 m) and the much 1994; Katoh et al. 1996). The major current features in deeper Japan/East Sea (depths exceed 2000 m). Korea/ the strait are the in¯ow of the warm Tsushima Current Tsushima Strait is about 330 km long and 100 m in from the East China Sea, the out¯ow into the Japan/ depth. The strait is about 160 km wide and is divided East Sea, and a counter¯ow on the east side of Tsushima by Tsushima Island into the East and West channels. Island (Miita and Ogawa 1984; Egawa et al. 1993). The current direction is generally northeastward along the * Naval Research Laboratory/Stennis Space Center Contribution channel, but southwestward countercurrents have been Number JA/7330/01/0067. observed (Katoh 1994; Isobe et al. 1994). Tidal currents with speeds approaching 50 cm s21 embedded in total currents of nearly 100 cm s21 have been reported in the Corresponding author address: Mr. W. J. Teague, Meso- and Fine- scale Ocean Physics Section, Naval Research Laboratory, Stennis strait by Isobe et al. (1994). Space Center, MS 39529-5004. Currents in the strait are generally thought to exhibit E-mail: [email protected] considerable seasonal variation in current velocity (Ega- q 2002 American Meteorological Society Unauthenticated | Downloaded 09/29/21 09:14 PM UTC 1622 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 32 FIG. 1. ADCP mooring locations, and bathymetry. Depths are in meters. Cheju Strait is located between Cheju-Do and Korea. The Korea/Tsushima Strait is located between Japan and Korea. Bathymetry is from the 1999 Digital Atlas for Neighboring Seas of Korean Peninsula (provided by Prof. Byung-Ho-Choi, Laboratory for Coastal and Ocean Dynamics Studies, Sungkyunkwan University). wa et al. 1993). Due to the scarcity of long-term direct channel but also is present during all seasons. Little current measurements, particularly in the winter season, seasonal variation is observed but largest current speeds this variability has been deduced from seasonal varia- are observed in October through December time period tions in water mass properties and in the sea level dif- with maximum currents in the near-surface layer in De- ferences among tidal stations along the Korean and Jap- cember and at middepth in November. anese coasts. Many of the direct current measurements The development of trawl resistant mounts for moor- are made using ship-mounted acoustic Doppler current ing instruments on the bottom makes long-term mea- pro®lers (ADCPs) where removing ship motions and surements possible in the strait. This technique was used tides are often a large source of error. Large seasonal in the Yellow Sea to deploy ADCPs and pressure gauges variation in the currents (higher magnitudes in the sum- (Teague et al. 1998; Teague and Jacobs 2000). This mer than in the winter) have been deduced using the study analyzes data from 14 similar moorings in the geostrophic current calculation (Yi 1966) and the sea strait (Fig. 1). Most of these moorings were deployed level differences (Yi 1970; Kawabe 1982; Toba et al. for about ten months. Twelve of the moorings (N1±N6, 1982). According to these studies, currents are at a max- S1±S6) are located on sections nearly spanning the strait imum in summer±fall and at a minimum in winter± to the northeast and southwest of Tsushima Island. The spring in the western channel. However, similar studies other two moorings are located just north of Cheju Is- in the eastern channel have found little seasonal vari- land and between Korea and Tsushima Island. One pur- ability (Isobe et al. 1994). Egawa et al. (1993) analyzed pose of these measurements is to delineate the nontidal ship-mounted ADCP observations taken over a three- currents and thus gain a better understanding of the year period. They found that the main axis of the Tsush- dynamics in the strait. Using the ®rst ®ve months of ima Current exists in the channel west of Tsushima Is- these measurements, Perkins et al. (2000a) present a land for all seasons and that the current ¯owing east of short description of the current ¯ow, and Jacobs et al. Tsushima Island is slower than the current in the western (2001b) estimate transports of 2.5 and 2.9 Sv (Sv [ 106 Unauthenticated | Downloaded 09/29/21 09:14 PM UTC JUNE 2002 TEAGUE ET AL. 1623 m3 s21) through the northeastern and southwestern lines, 1995). The U.S. Naval Oceanographic Of®ce imple- respectively. Along the southwestern section, nontidal mented similar moorings in 1995 in the Yellow Sea ¯ows exceed 50 cm s21, and along the northeastern (Teague et al. 1998). Within the Korea/Tsushima Strait, section nontidal ¯ows exceed 70 cm s21. The two sec- 14 mooring packages were deployed in a combined ef- tions show markedly different ¯ow regimes. At the fort between the Naval Research Laboratory, the Naval southern entrance the ¯ow through the strait contains a Oceanographic Of®ce, and the Korea Ocean Research broad maximum at midchannel. The northeastern sec- and Development Institute (KORDI). Six moorings tion, is marked by strong spatial variability, but in the were deployed along each of two sections that span the mean consists of two streams, one on each side of the entire Korea/Tsushima Strait. One section is located strait. Between the two is a regime of highly variable northeast of Tsushima Island (mooring positions de- ¯ow with a weak mean, presumably due to the sepa- noted by N1±N6 in Fig. 1) and the other is located ration of the ¯ow by Tsushima Island. Tides are found southwest of Tsushima Island (denoted by S1±S6 in Fig. to signi®cantly contribute to the total current ¯ow and 1). The deployment times generally cover the period are described by Teague et al. (2001). Total currents from May 1999 through March 2000. Moorings along with tides can be twice as large as the currents without the two sections were recovered and redeployed in Oc- tides. Inertial currents dynamics within the strait, par- tober 1999. Addition moorings were deployed in the ticularly important during summer, are examined by Ja- strait between Tsushima Island and Korea (C1) from cobs et al. (2001a). October 1999 through March 2000 and in Cheju Strait Numerous studies have proposed different theories (K1) from March 1999 through December 1999. Geo- on the dynamics of the generation mechanism and sea- graphical positions, deployment periods, water depths, sonal variation of the Tsushima Current. The low fre- and ADCP sample levels are provided in Table 1. quency current variations are expected to be mainly re- Each deployed package consists of an ADCP and a lated to the sea level drop between the East China Sea wave/tide gauge (except at C1 and K1) housed in a and the Japan/East Sea and ultimately linked to the me- trawl-resistant bottom mount (TRBM).
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